Methods and appraratus for ground fault circuit interrupt detection using a single transformer

A ground fault circuit interrupt (“GFCI”) device for use with an AC power system that includes a line conductor, a neutral conductor and a transformer, the line conductor and the neutral conductor each coupled between a source and a load, the neutral conductor coupled to ground at the source, wherein the transformer includes a first primary winding coupled in series with the line conductor, a second primary winding coupled in series with the neutral conductor, and a secondary winding, the device comprising:

• a ground current fault detector circuit coupled to the secondary winding, wherein the ground current fault detector circuit provides a first detection signal if a current in the secondary winding exceeds a first predetermined threshold;
• and
• a grounded neutral fault detector circuit coupled to the secondary winding, wherein the grounded neutral fault detector circuit:
• (a) includes an AC signal source that drives the secondary winding with an AC voltage at a specified amplitude and a specified frequency, (b) monitors a load current in the secondary winding, and (c) provides a second detection signal if the load current exceeds a predetermined threshold.